A Technical Guide For Static Headspace Analysis Using GC
Technical GuideA Technical Guidefor Static HeadspaceAnalysis Using GCInside:Basic Principles of HeadspaceAnalysisInstrumentationSystem Optimization(Troubleshooting)Headspace ApplicationsRecommended Headspace AnalysisProducts
2Table of ContentsIntroduction . 2Basic Principles of HeadspaceAnalysis . 3 Partition Coefficient Phase Ratio Combining K and β Derivatization/ReactionHeadspace Headspace Sample SizeInstrumentation . 6 Gas-Tight Syringe Injection Balanced-Pressure System Pressure-Loop SystemSystem Optimization(Troubleshooting) . 8 Sample Preparation Sample Vial Sample Vial Heater and Mixer Sampling Transfer Line Injection Port InterfaceHeadspace Applications . 11 Blood Alcohol Analysis USP 467 European Pharmacopoeia TestsRecommended Headspace AnalysisProducts . 15 Capillary Columns Guard Columns Press-Tight Connectors Analytical Reference Materials GC Accessories For technical support, call800-356-1688, ext. 4(814-353-1300, ext. 4)or call your localRestek representative.www.restekcorp.comStatic headspace gas chromatography (GC) is a technique used for the concentration and analysis of volatile organic compounds. This technique is relativelysimple and can provide sensitivity similar to dynamic purge and trap analysis. Thepopularity of this technique has grown and has gained worldwide acceptance foranalyses of alcohols in blood and residual solvents in pharmaceutical products.Other common applications include industrial analyses of monomers in polymersand plastic, flavor compounds in beverages and food products, and fragrances inperfumes and cosmetics.Sample matrices like blood, plastic, and cosmetics contain high molecular weight,non-volatile material that can remain in the GC system and result in poor analyticalperformance. Many laboratory analysts use extensive sample preparation techniquesto extract and concentrate the compounds of interest from this unwanted nonvolatile material. These extraction and concentration techniques can become timeconsuming and costly. Static headspace analysis avoids this time and cost by directlysampling the volatile headspace from the container in which the sample is placed.Because of the diversities in the industry and related products, this guide attempts tocover only the basic principles of static headspace and demonstrate how to applythem to achieve optimum chromatographic results. With an understanding of theseprinciples, various instrumentation will then be reviewed to help build upon thisknowledge and identify the benefits and potential problems associated with eachmode of sample transfer. Information from the Basic Principles and Instrumentationsections of this guide can then be brought together and applied to the conditions andmethodology of common analyses. Like most applications, a variety of problemsmay arise in which the System Optimization section will help to identify theseproblems and offer techniques to help resolve them.Time and money are two of the many reasons why an analystwould use static headspace analysis. Other reasons may includeease of operation and the ability to assay a variety of samplematrices.
3Basic Principles of Headspace AnalysisMost consumer products and biological samples are composed of a wide variety ofcompounds that differ in molecular weight, polarity, and volatility. For complexsamples like these, headspace sampling is the fastest and cleanest method foranalyzing volatile organic compounds. A headspace sample is normally prepared ina vial containing the sample, the dilution solvent, a matrix modifier, and theheadspace (see Figure 1). Volatile components from complex sample mixtures canbe extracted from non-volatile sample components and isolated in the headspace orvapor portion of a sample vial. An aliquot of the vapor in the headspace is deliveredto a GC system for separation of all of the volatile components.In order to achieve the best performance when using headspace/GC, careful attention should be used in sample preparation and instrument setup. Key issues toaddress when setting up headspace/GC systems include minimizing system deadvolume, maintaining inert sample flow paths, and achieving efficient sampletransfer. These issues, as well as other instrument setup-related topics, are addressedlater in the System Optimization section of this guide.Figure 1Phases of the headspace vial.G the gas phase (headspace).The gas phase is commonly referred to as the headspaceG and lies above the condensed sample phase.volatileanalytes}}sample, dilutionsolvent, and matrixmodifierS the sample phase.The sample phase contains the compound(s) of interestand is usually in the form of a liquid or solid in combinaS tion with a dilution solvent or a matrix modifier.Once the sample phase is introduced into the vial and thevial is sealed, volatile components diffuse into the gasphase until the headspace has reached a state of equilibrium as depicted by the arrows. The sample is then takenfrom the headspace.Partition CoefficientSamples must be prepared to maximize the concentration of the volatile componentsin the headspace, and minimize unwanted contamination from other compounds inthe sample matrix. To help determine the concentration of an analyte in theheadspace, you will need to calculate the partition coefficient (K), which is definedas the equilibrium distribution of an analyte between the sample phase and the gasphase (Figure 2).Compounds that have low K values will tend to partition more readily into the gasphase, and have relatively high responses and low limits of detection (Figure 3). Anexample of this would be hexane in water: at 40 C, hexane has a K value of 0.14 inan air-water system. Compounds that have high K values will tend to partition lessreadily into the gas phase and have relatively low response and high limits ofdetection. An example of this would be ethanol in water: at 40 C, ethanol has a Kvalue of 1355 in an air-water system. Partition coefficient values for other commoncompounds are shown in Table I.Figure 2K and β are important variables inheadspace analysis.Equation 1Partition Coefficient (K) Cs/CgEquation 2Phase Ratio (β) Vg/VsCs concentration of analyte in sample phaseCg concentration of analyte in gas phaseVs volume of sample phaseVg volume of gas phaseTable IK Values of Common Solvents in AirWater Systems at 40 thanen-butyl acetateethyl acetatemethyl ethyl ketonen-butanolisopropanolethanoldioxaneK 564782513551618
4Figure 3Sensitivity is increased when Kis minimized.(Ideal)CgKFigure 4Sensitivity is increased when βis minimized.(Ideal)CgβK can be lowered by changing theTable IItemperature at which the vial is equiliCommonsaltsused to decreasebrated or by changing the composition ofmatrixeffects.the sample matrix. In the case of ethanol,K can be lowered from 1355 to 328 byammonium chlorideraising the temperature of the vial fromammonium sulfate40 C to 80 C. It can be lowered evensodium chloridesodium citratefurther by introducing inorganic salt intosodium sulfatethe aqueous sample matrix. High saltpotassium carbonateconcentrations in aqueous samplesdecrease the solubility of polar organicvolatiles in the sample matrix and promote their transfer into the headspace,resulting in lower K values. However, the magnitude of the salting-out effect on K isnot the same for all compounds. Compounds with K values that are already relatively low will experience very little change in partition coefficient after adding asalt to an aqueous sample matrix. Generally, volatile polar compounds in polarmatrices (aqueous samples) will experience the largest shifts in K and have higherresponses after the addition of salt to the sample matrix. Table II lists most of thecommon salts used for salting-out procedures.Phase RatioThe phase ratio (β) is defined as the relative volume of the headspace compared tovolume of the sample in the sample vial (Figure 2). Lower values for β (i.e., largersample size) will yield higher responses for volatile compounds (Figure 4). However, decreasing the β value will not always yield the increase in response needed toimprove sensitivity. When β is decreased by increasing the sample size, compoundswith high K values partition less into the headspace compared to compounds withlow K values, and yield correspondingly smaller changes in Cg. Samples that containcompounds with high K values need to be optimized to provide the lowest K valuebefore changes are made in the phase ratio.Combining K and βPartition coefficients and phase ratioswork together to determine the finalconcentration of volatile compounds inthe headspace of sample vials. Theconcentration of volatile compounds inthe gas phase can be expressed asCg Co/(K β) (where Cg is the concentration of volatile analytes in the gasphase and Co is the original concentration of volatile analytes in the sample).Striving for the lowest values for bothK and β will result in higher concentrations of volatile analytes in the gasphase and, therefore, better sensitivity(Figure 5). For customer service, call800-356-1688, ext. 3(814-353-1300, ext. 3)www.restekcorp.comor call your localRestek representative.Figure 5Lower K and β result in higher Cgand better sensitivity.(Ideal)CgK and β
5Derivatization/Reaction HeadspaceDerivatization is another technique that can be used to increase sensitivity andchromatographic performance for specific compounds. Compounds such as acids,alcohols, and amines are difficult to analyze because of the presence of reactivehydrogens. When attempting to analyze these types of compounds, they can reactwith the surface of the injection port or the analytical column and result in tailingpeaks and low response. In addition, they may be highly soluble in the samplephase, causing very poor partitioning into the headspace and low response.Derivatization can improve their volatility, as well as reduce the potential for surfaceadsorption once they enter the GC system.Common derivatization techniques used in reaction headspace/GC are esterification,acetylation, silylation, and alkylation. Any of these derivatization techniques can beperformed using the sample vial as the reaction vessel (see Table III for a list ofcommonly used reagents). Although derivatization may improve chromatographicperformance and volatility for some compounds, derivatization reactions mayintroduce other problems into the analytical scheme. Derivatization reagents as wellas the by-products from derivatization reaction may be volatile and can partition intothe headspace along with derivatized compounds. These extra volatile compoundsmay pose problems by eluting with similar retention times as the compounds ofinterest, causing either partial or complete coelutions.Derivatization reactions also are typically run at elevated temperatures. Pressuresinside the sample vial may exceed the pressure handling capabilities of the vial orthe septa. Specially designed septa are available that allow excess pressure to bevented during derivatization reactions.Table IIICommon reagents used to derivatize compounds of interest.Compound of Interestfatty acidsglycerolDerivatizing Reagentmethanolwith boron trifluorideacetic anhydridewith sodium carbonateResulting DerivativeesterificationFor more information on headspaceanaysis, check out the textbook,Static Headspace-GasChromatography, Theory and Practiceby Bruno Kolb and Leslie S. Ettre.acetylationFor more information on derivatization, please refer to the “Handbook of AnalyticalDerivatization Reactions” by Daniel R. Knapp or to the text at right.Headspace Sample SizeIn addition to working with K, β, and derivatization reactions, sensitivity also can beimproved by simply increasing the size of the headspace sample that is withdrawnfrom the sample vial and transferred to the GC. Increasing the sample size alsomeans that the amount of time it takes to transfer the sample to the column willincrease in proportion to the column volumetric flow rate. Sample size can beincreased only to the point that increases in peak width, as a result of longer sampletransfer times, will not affect chromatographic separations. Larger sample sizes andlonger transfer times can be offset by using cryogenic cooling and sample refocusing at the head of the column.www.restekcorp.com
6InstrumentationGas-Tight Syringe InjectionUse of a gas-tight syringe autosampling system is one of three common techniques(gas-tight syringe, balanced pressure, and pressure loop) used to transfer aheadspace sample. Most of the autosampling units can retrofit to a standard GC witha split/splitless injection port, making them relatively simple to use and understand.These systems do not require the use of special configurations or special instrumentation other than the autosampler itself. The gas-tight syringe autosampler isbeneficial for use with diverse samples because of the variety of sampler configurations and method options available.The gas-tight syringe technique operates by initially thermostatting the sample in anincubation oven at a given temperature and for a given time until it has reached astate of equilibrium (Figure 6, Step 1). Once the sample has reached an equilibrium,an aliquot is taken from the headspace using the gas-tight syringe (Figure 6, Step2), and the aliquot is injected into the GC as if it were a liquid sample injection(Figure 6, Step 3).Figure 6: Gas syringe systemStep 1Sample reachesequilibriumFigure 7Gas-tight syringe autosamplerTRACE HS850www.restekcorp.comStep 2Sample is extracted fromheadspaceStep 3Sample is injectedSeveral concerns exist regarding this technique. Because the sample is being transferred from a heated oven, the syringe also must be heated to ensure that the sample willnot recondense in the syringe. Many manufacturers have taken this into considerationand their samplers now come with a heated syringe assembly. There also are reproducibility issues because of possible sample loss. As the sample is transferred from the vialto the injection port, some of it may be lost because of the pressure differences betweenthe vial and atmospheric conditions. Beyond these concerns, the gas-tight syringetechnique is simple to use, can retrofit into a variety of GC systems, and is best suitedfor diverse samples. Examples of manufacturers and models of the gas-tight syringeunits are: the ThermoQuest TRACE HS2000 and HS850 (Figure 7) HeadspaceAutosamplers and the Leap Technologies CTC COMBI PAL Sampler.Balanced-Pressure SystemAnother common technique is the balanced-pressure system, which is capable ofgenerating results with a high degree of repeatability. It uses a seamless injectiondirectly from the vial into the carrier gas stream without additional moving parts otherthan a valve and a needle. The balanced-pressure system, like other techniques, usesan incubation oven to thermostat the vial so the sample reaches equilibrium (Figure 8,Step 1). During these initial steps, a needle is inserted into the vial and then ispressurized with a carrier gas (Figure 8, Step 2). After the vial is pressurized andequilibrium has been reached, the valve is switched for a specific amount of time toredirect the sample into the transfer line and onto the column (Figure 8, Step 3).
7Figure 8: Balanced-pressure systemStep 1Sample reachesequilibriumStep 2Pressurization of injectionStep 3Sample is extracted andinjectedFigure 9Balanced-pressure autosamplerPerkin-Elmer HS 40XLoutletinlet/outletBecause this technique uses a theoretical amount of time to inject the sample, theabsolute volume of the sample is unknown. However, this technique is highly reproducible because the number of moving parts are minimized, which decreases thechance for compound adsorption and loss via leaks. An example of a balancedpressure system is the HS 40XL manufactured by Perkin-Elmer (Figure 9).Pressure-Loop SystemThe last common injection technique discussed in this guide is the pressure-loopsystem. Unlike balanced-pressure, the pressure-loop system uses a known amount ofsample. This technique typically uses a six-port valve, and initially thermostats andpressurizes the vial as in the previously described techniques (Figure 10,Step 1). After pressurization, the valve is turned and the loop is filled with thesample (Figure 10, Step 2). After the loop has been filled, the valve is turned again toredirect the gas flow and flush the sample into the transfer line leading to the analytical column (Figure 10, Step 3).Figure 10: Pressure-loop systemStep 1Sample reachesequilibrium/pressurizationStep 2Sample is extracted fromheadspaceStep 3Sample is injectedFigure 11Pressure-loop systemOI Model 4632The pressure-loop system has several advantages and disadvantages. One of theadvantages of this system is that the loop can be thermostatted to high temperatures,which helps to lessen adsorption of higher molecular weight and sensitive compounds. The fixed volume of the sample loop also helps to improve run-to-runreproducibility. A disadvantage of a pressure-loop system is that it may cause ghostpeaks because of sample carryover from a previous analysis.1 Several makes andmodels of pressure-loop systems include the OI Model 4632 (Figure 11), VarianGenesis, Tekmar 7000HT, and the HP 7694E.
8System Optimization (Troubleshooting)Chromatographic performance in Headspace/GC is greatly influenced by how thesample is introduced into the analytical column. Variables that affect samplepreparation and transfer of the sample from the headspace unit to the analyticalcolumn must be optimized to obtain reproducible and efficient separations. Keyissues to address when setting up headspace/GC systems include minimizing systemdead volume, maintaining inert sample flow paths, and achieving efficient sampletransfer. This section will explain how to optimize areas that are critical in addressing these issues and providing good chromatographic performance.Sample PreparationSamples for headspace/GC must be prepared in such a manner as to maximize theconcentration of the volatile sample components in the headspace while minimizingunwanted contamination from other compounds in the sample matrix. Samplematrices such as biological samples, plastics, and cosmetics can contain highmolecular weight, volatile material that can be transferred to the GC system. Waterfrom the sample matrix also can cause problems by recondensing in the transfer line.Incomplete or inefficient transfer of high molecular weight compounds or watervapor from sample matrices can produce adsorptive areas in the transfer line orinjection port that can lead to split peaks, tailing peaks, or irreproducible responsesor retention times. To minimize matrix problems and prevent water condensationfrom aqueous samples, use a higher transfer line temperature ( 125 C–150 C).High-concentration samples need to be prepared appropriately to obtain optimalchromatography. High-concentration samples can produce ghost peaks in subsequent analyses due to carryover of sample from previous injections. Samplecarryover can be minimized by using higher transfer line and injection port temperatures, but some samples may need to be diluted and re-analyzed to obtain reliableresults. Additionally, we recommend injecting standards and samples in order fromlow to high concentrations to help minimize carryover. When sample carryover orghost peaks are evident, you may need to bake-out the column at its maximumoperating temperature and elevate the transfer line temperature in order to removeall of the residual sample. If high-concentration samples are anticipated in asequence of samples, running a blank after the suspected samples will reducecarryover contamination of following ones. It is good lab practice to handle standards and method blanks the same way samples are handled to make any vial orsample preparation problems easier to identify. Always use pre-cleaned vials forsample preparation and storage.www.restekcorp.comSample VialSample vials should be selected to match the type and size of the sample beinganalyzed. Always use pre-cleaned vials for sample preparation and storage. Vialsthat are not properly cleaned prior to packaging or that absorb contaminants duringshipping can produce unknown chromatographic peaks, or “ghost peaks.” Ghostpeaks that are the result of vial contamination can be identified by running methodblanks and zero standards during the system calibration sequence.The septa used to seal the headspace vials also can be a source for contaminants,which can bleed into the headspace of the vial during equilibration. These contaminants can appear as single peaks or multiple peak patterns. Some septa are availablewith a PTFE face to eliminate bleed from the rubber portion of the septa. Thesesepta should not be re-used. Once the PTFE face has been punctured by a syringe,contaminants from the rubber portion of the septa can migrate into the headspaceand show up as unidentified peaks. Again, the use of method blanks can help todetermine the source of contaminants.
9Sample Vial Heater and MixerOnce the sample is placed inside a clean, non-contaminating vial and the vial issealed, volatile compounds from the sample will partition into the headspace until astate of equilibrium is reached. The rate at which volatile compounds partition out ofthe sample matrix and into the headspace, as well as the equilibrium concentrationof volatile compounds in the headspace depends on several parameters (see alsoIntroduction of this guide). Temperature, time, and mixing can beused to improve the transfer of volatileanalytes from the sample into theheadspace of the vial. Adjusting thetemperature of the sample will changethe solubility of the analyte in thesample matrix and can be used to drivethe equilibrium in favor of the gasphase. Sufficient time must be built intothe sample cycle in order to achieve aconstant state of equilibrium. Some sample matrices require longer equilibrationtimes due to physical characteristics like high viscosity. Shaking or vibrating the vialduring heating can assist in achieving equilibrium more quickly by exposing moresample surface area for the transfer of volatile analytes to the headspace.Shaking or vibrating thevial during heating canassist in achievingequilibrium.SamplingThere are several techniques used to transfer samples from the vial to theGC. When using a gas-tight syringe for sampling, heat the syringe to atemperature comparable to the sample vial temperature. This minimizespressure differences and condensation problems. To prevent carryover frominside the syringe, flush the syringe after each injection. Because gas-tightsyringe samplers inject through the GC injection port septum, ensure theseptum is well maintained to decrease the possibility of a leak.For balanced-pressure sampling instruments, analysts should consider theinertness and efficiency of the components that make up the sample pathwayinside the autosampler. If sensitive compounds are being analyzed, an inertpathway should be used to decrease possible adsorption. Materials such asstainless steel, nickel, Silcosteel and PTFE coatings, or KEL-F parts canbe used to minimize sample adsorption and peak tailing. Transfer line internaldiameter should be as narrow as possible to help maintain narrow sample bandwidths and symmetrical peak shapes (see the following optimization of transfer linesfor more information). Analysts also should ensure that balanced-pressure instruments are leak-free and operate with the least amount of dead volume in the sampleflow path. This will help obtain optimal peak shape and sensitivity.When using pressure-loop sampling instruments, the same concerns apply as withgas-tight syringe and balanced-pressure systems. Inert sample pathways and lowdead volume systems will yield the best chromatographic performance. In pressureloop systems, a gas sampling valve with a sample loop is used to transfer the samplefrom the headspace unit to the GC. Adequate purging of the sample valve and loopwill guard against sample carryover. If low response or broad peaks are observed, itmay be necessary to increase the sample vial pressure to ensure that the sample loopis being completely filled with headspace sample. If there are extraneous peakspresent due to carryover of matrix contaminants, increase the sample valve temperature to prevent sample carryover, condensation, and contamination. For technical support, call800-356-1688, ext. 4(814-353-1300, ext. 4)or call your localRestek representative.www.restekcorp.com
10 Use an inert transferline when optimizingpressure-loop systems.Transfer LineAfter the headspace sample is withdrawn from the vial, it ready to be transferred tothe GC. In balanced-pressure and pressure-loop systems a short piece of tubingcalled a transfer line is used to transfer the sample from the autosampler to the GC.Transfer line material must be chosen that suits the sample analytes. Many differentmaterials can be used as transfer line tubing, including stainless steel, nickel, fusedsilica, and Silcosteel - or Siltek -coated tubing. Stainless steel provides a strong,flexible tubing material, but can be adsorptive towards more active analytes such asalcohols, diols, and amines. Nickel and Silcosteel tubing are highly inert towardsactive compounds and provide ruggedness similar to stainless steel. Fused silica andSiltek tubing are extremely inerttowards active compounds, howeverthey are not as rugged as nickel orSilcosteel tubing.The internal diameter of thetransfer line should be chosendepending on the internaldiameter of the analyticalcolumn, the column flow rate,and the flow rate deliveredfrom the autosampler. To eliminate tubing dead volume, use the smallest diameter tubing possible. Forexample, compound residence time in a 1.0mm ID transfer line is 3.6 times greaterthan in the same length of 0.53mm ID tubing. Reducing the residence time of theheadspace sample in the transfer line helps to minimize band broadening. Therefore,the flow rate should be set as high as possible to quickly move the sample cloudthrough the tubing and minimize any dead volume effects.Transfer line temperature should be set depending on the analytes of interest and thesample matrix. Typical transfer line temperatures range from 80 C to 125 C. Tominimize matrix problems and prevent water condensation from aqueous samples,use a higher transfer line temperature ( 125 C to 150 C).Restek’s technical service is hereto help. If you still have questionsafter reviewing this guide,please call us at 800-356-1688or 814 353-1300, ext. 4, or callyour local Restek representative.www.restekcorp.comInjection Port InterfaceThe quality of the connection of the transfer line to the analytical column greatlyaffects sample bandwidth. In most cases, the transfer line has a smaller internaldiameter than the injection port liner, and the vaporized headspace sample carryingthe compounds of interest will be diluted into a larger volume of carrier gas whenthe sample elutes from the transfer line into the inlet liner. This can lead to broaderpeaks, tailing peaks, lower sensitivity, and loss of resolution. Because headspacesamples are already in a gaseous state (vapor cloud) when they enter the injectionport, there is no need to use a large buffer volume in the liner to allow for sampleexpansion as when analyzing liquid samples. Using injection port liners that havesmaller internal diameters and lower buffer volumes will help maintain a narrowbandwidth as samples move from the end of the transfer line to the head of theanalytical column. 1.0mm ID deactivated injection port liners are recommended formost headspace applications to achieve the lowest injection port dead volume.If band-broadening due to excess dead volume in the system is still a problem, peakshape may be improved by refocusing sample analytes at the analytical column head.Highly volatile compounds can be trapped at the column head and refocused into anarrow bandwidth by reducing the initial oven temperature below the boiling point ofcompounds of interest. After the sample is completely transferred to the column, theoven temperature can be increased to move the compounds through the column.
11Headspace ApplicationsBlood Alcohol AnalysisAnalysis time and resolution are two critical factors when developing a GC assay forethanol. Analysis time for each sample should be as short as possible while stillmaintaining baseline resolution for all analytes. Isothermal analysis is the method ofchoice because it eliminates the cool-down period between temperature-programmedruns. Overall analysis time can be reduced in isothermal analysis by raising the oventemperature or by increasing carrier gas flow rate. However, in attempting to shortenthe analysis time, either by increasing the flow rate or raising the temperature, manytraditional capillary column stationary phases fail to provide adequate resolution ofall the components commonly tested during blood alcohol analysis. Current advanceshave aided in the design of two novel capillary column stationary phases to meet all ofthese requirements—the Rtx -BAC1 and the Rtx -BAC2 columns.Quantitation Technique for BloodAlcohol Analysis (Internal Standard)The internal standard technique uses one ormore designated compounds at knownconcentrations spiked into the sample. Theresponse of the compounds of interest arethen compared to the results of the internalstandard. There are several advantages tothis technique. Multiple injections of thestandard are not necessary for concentration calculations; small changes in injectionvolumes or detector response over time canbe determined.Figure 12
address when setting up headspace/GC systems include minimizing system dead volume, maintaining inert sample flow paths, and achieving efficient sample transfer. These issues, as well as other
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